1. Field
This patent specification relates to analyzing subsurface electromagnetic survey data. More particularly, this patent specification relates to methods and systems for analyzing electromagnetic survey data obtained using receivers and/or transmitters in a conductive casing lined borehole. Even more particularly, this patent specification relates to methods and systems for improved analysis of such survey data through inversion with constrained casing correction coefficients.
2. Background
Cross borehole (CB), surface-to-borehole (STB), and borehole-to-surface (BTS) and single well electromagnetic (EM) imaging surveys include a series of electric or magnetic dipole sources that are energized in a borehole, on the surface of the earth, or near the seafloor, and measurements of magnetic and/or electric fields are made in a different borehole, or on the earth's surface or seafloor.
The purpose of the surveys are typically to provide an image of formation resistivity, including the location of high resistivity zones such as hydrocarbon bearing reservoirs, gas injection zones, and fresh-water aquifers, and conductors such as produced by the presence of saline water, mineralized zones, and shale and clay. These images are used for applications such as but not limited to the characterization of hydrocarbon reservoirs and ground-water aquifers, for locating bypassed oil deposits, for monitoring reservoir production, groundwater, steam, and gas injection processes, and for imaging saltwater intrusion into aquifers.
One issue related to surveying methods that are presently used for these electromagnetic imaging techniques is the effect of steel casing on the EM fields measured or broadcasted by borehole sensors. The attenuation induced by the conductive casing requires a correction to be applied so only the formation signal is being inverted for. Such methods of casing effect removal have been detailed in the literature review contained in U.S. Pat. No. 6,294,917 (hereinafter “the '917 Patent”) and U.S. Pat. No. 7,565,244 (hereinafter “the '244 Patent”), and in U.S. Patent Application Nos. 2009/0005993(hereinafter “the '993 Patent Application”) and 2009/0157320 (hereinafter “the '320 Patent Application”), all four of which are hereby incorporated by reference herein. The '244 Patent, and the '993 and '320 Patent Applications also provide a method for calculating multiple beta (β coefficients (or casing attenuation coefficients) by treating beta essentially as a model parameter to be inverted.
However, in some situations it has been found that the above described beta inversion techniques may produced unsatisfactory results. In particular, when solving for a parameter such as resistivity in some applications the resistivity values produced close to the borehole may be unacceptably biased. Thus, there is a need in some applications for an improved technique.